Optical materials for lenses, backlights, light guide plates and optical films are naturally required to have high transparency, but there are increasing demands for heat resistance, low water absorption, low dielectric constant, flexibility and toughness.
Known transparent resins used in optical materials are for example polyester resins, acrylic resins, polycarbonate resins, polyacrylate resins and polyethersulfone resins. These resins, however, have insufficient heat resistance, water absorption properties and transparency.
A great number of hydrogenated ring-opened polymers or addition polymers of cyclic olefin compounds are developed as resins having excellent transparency and heat resistance. Because the main chains of these resins are formed of alicyclic hydrocarbons, they have lower absorption in a low wavelength region than aromatic resins.
Many kinds of ring-opened cyclic olefin polymers and hydrogenated products thereof are reported as useful optical materials for the production of lenses or optical disks (for example, Patent Documents 1, 2, 3, 4, 5 and 6). The ring-opened cyclic olefin (co)polymers and hydrogenated products thereof disclosed in these documents have excellent heat resistance, low water (moisture) absorption, good optical properties such as transparency, and high forming properties such as injection moldability.
Further, ring-opened polymers from cyclic olefin monomers having a polar group in the molecule, and hydrogenated products thereof are disclosed (for example, Patent Documents 7 and 8). They have superior heat resistance, optical properties, forming properties and affinity for other materials, and are excellent in post workability such as bonding. However, ring-opened cyclic olefin polymers have a double bond in the polymer main chain and entail hydrogenation to improve heat degradation resistance. As a result, industrial productivity and costs are deteriorated.
It is known that addition polymerization of cyclic olefin compounds gives resins having excellent heat resistance and transparency. Because the polymer main chain has no double bonds, the polymers have high heat degradation resistance and do not entail hydrogenation, and good industrial productivity and cost advantages are obtained. A number of cyclic olefin addition polymers are reported. For example, Patent Documents 9 and 10 disclose addition copolymers of cyclic olefin compounds and α-olefins. In these copolymers, however, a series of structural units derived from α-olefin such as ethylene is sometimes crystallized and the transparency is lowered. Such materials are often not suited for use as optical materials. Further, cyclic olefins and α-olefins greatly differ from each other in polymerization reactivity and consequently the copolymers often have a nonuniform composition and lower transparency.
On the other hand, cyclic olefin addition polymers consisting solely of structural units derived from cyclic olefin compounds are produced with titanium catalysts, zirconium catalysts, cobalt catalysts, nickel catalysts and palladium catalysts, and they show very high heat resistance and transparency as known in the art. It is known that the selection of catalyst determines the polymer's stereoregularity (atactic, erythro-di-syndiotactic, erythro-di-isotactic), addition polymerization mode (addition at 2 and 3 positions, addition at 2 and 7 positions), and molecular weight controllability. For example, norbornene polymers produced with a zirconium metallocene catalyst are non-soluble and do not dissolve in general solvents (Non-Patent Document 1). Norbornene addition polymers produced with a nickel catalyst show good solubility in hydrocarbon solvents such as cyclohexane (Patent Document 11) but are poor in mechanical strength and are brittle (Patent Document 12).
Patent Documents 12, 13 and 14 report that a specific catalyst containing a palladium compound shows high polymerization activity and provides cyclic olefin addition polymers having excellent transparency, heat resistance and mechanical strength. Further, Patent Document 15 teaches that addition copolymerization of a hydrolyzable silyl group-containing cyclic olefin with a catalyst containing a palladium compound affords a cyclic olefin copolymer showing excellent heat resistance and dimension stability. The addition polymers described in these documents have very high heat resistance; however, they cannot be shaped by thermal fusion and forming methods are limited to solution casting methods. The casting methods involve large amounts of solvents, and entail solvent removal and collection and also entail larger facility therefor, thereby lowering productivity and increasing costs.
To lower the glass transition temperature of the cyclic olefin addition polymers and thereby render the polymers melt-formable, it is proposed that alkyl-substituted cyclic olefin compounds may be used as monomers. For example, Patent Document 16 describes addition copolymers having 5-hexyl-2-norbornene. Patent Document 11 and Non-Patent Document 2 describe that a norbornene having a long-chain alkyl group is used as a monomer and the glass transition temperature of an addition copolymer is controlled by changing the chain length and proportion of the norbornene monomers. However, these documents do not describe effects of polymerization catalysts on mechanical strength of obtainable shaped articles. Further, the polymerization catalysts used in the above documents are still insufficient in activity, and post treatments for removing residual unreacted monomers and catalysts are required.
Furthermore, a cyclic olefin having a long-chain alkyl group shows lower polymerization reactivity than norbornene, and copolymerization of these compounds results in a copolymer having a nonuniform composition. Patent Documents 16 and 11 and Non-Patent Document 2 are silent on different reactivity of monomers and composition distribution. The composition distribution becomes more nonuniform as the conversion increases, and the obtainable shaped articles tend to have lower transparency and strength. Accordingly, achieving both high conversion and high transparency is desired. That is, it is desired that economic and productive processes are developed to produce cyclic olefin copolymers having good heat resistance and melt-formability as well as high transparency at high conversion. However, there have been no reports of such production processes.    Patent Document 1: JP-A-S63-21878    Patent Document 2: JP-A-H1-138257    Patent Document 3: JP-A-H1-168725    Patent Document 4: JP-A-H2-102221    Patent Document 5: JP-A-H2-133413    Patent Document 6: JP-A-H4-170425    Patent Document 7: JP-A-S50-111200    Patent Document 8: JP-A-H1-132626    Patent Document 9: JP-A-S61-292601    Patent Document 10: U.S. Pat. No. 2,883,372    Patent Document 11: JP-B-H9-508649    Patent Document 12: JP-A-2006-52347    Patent Document 13: JP-A-2005-162990    Patent Document 14: JP-A-2005-213435    Patent Document 15: JP-A-2005-48060    Patent Document 16: JP-A-H8-198919    Non-Patent Document 1: Makromol. Chem. Macromol. Symp., Vol. 47, 831 (1991)    Non-Patent Document 2: Proc. Am. Chem. Soc. Div. Polym. Mater.: Sci. Eng. Vol. 75, 56 (1997)